Paper mills have for many years made extensive use, for the cleaning of paper making stock, of screening apparatus embodying a cylindrical perforate screen member defining supply and accepts chambers on the opposite sides thereof in a closed housing and provided with a rotor member which operates in one of the chambers to keep the screen perforations open and free from solid material tending to cling to the screen surface. Commonly, the stock or furnish is delivered to the supply chamber adjacent the end of the screen member, and the material rejected by the screen member is collected and discharged from the opposite end of the screen member.
The assignee of this invention has manufactured and sold many such screens, originally in accordance with Staege U.S. Pat. No. 2,347,716, and more recently in accordance with Martindale U.S. Pat. No. 2,835,173, the latter construction being characterized by a rotor comprising bars or vanes of air-foil section in closely spaced but non-contacting relation with the surface of the screen member. More specifically, these vanes have been moved along the screening surface at relatively low speeds, e.g. in the range of 1,250 - 2,500 feet per minute, with the clearance between the supply side of the screen member and the nearest portion of the vanes being in the range of 0.030 - 0.060 inch.
The art has experimented widely with detailed variation in screens of the above type, including variations in the vane shape and other forms of rotor, and in the size, spacing and configuration of the perforations in the screen member. For example, such screens have recently been offered to the trade wherein the rotor is a wall member provided with multiple bumps or other offset portions over its surface for the purpose of creating localized changes in volume and resulting localized agitation effects in the annular space between the rotor and the screen member, a typical such construction being shown in Clarke-Pounder U.S. Pat. No. 3,363,759. The variations in the perforations in the screen member have tended in recent years to result in screen members provided with elongated slots rather than round holes, typical such constructions being shown in Lamort U.S. Pat. Nos., 3,161,708 and 3,174,622 and Holz U.S. Pat. No. 3,581,983.
The above noted Seifert patent and application disclose screens generally of the same construction as in the Martindale patent, incorporating both slotted (U.S. Pat. No. 3,849,302) and circularly perforated (Ser. No. 496,160) screen members, and having rotors especially constructed to maintain the vanes spaced away from the screen member on its inlet side by a sufficient distance to establish a tubular layer of stock of substantial thickness adjacent the inlet surface of the screen member. In the use of those screens, the rotor is operated at relatively high speeds to develop strong hydraulic shear forces in the tubular layer of stock adjacent the screen member which cause tangential orientation of predominantly two-dimensional contaminant particles causing them to flow past rather than through the screen perforations. In addition, the stock is continuously recirculated in the supply chamber to prevent undue increase in the consistency of the stock in the tubular layer, thereby, providing for screening at high consistencies as well as high throughput rates.
Screens constructed in accordance with the Seifert patent and application have proved very successful in handling reasonably clean stock, but they have encountered problems in the handling of relatively dirty stock, and particularly stock having multiple stringy contaminants such, for example, as stock containing fiber salvaged from municipal waste as disclosed in U.S. Pat. No. 3,736,223. The stringy material present in such stocks has a tendency to collect on the rotor, particularly by folding over or wrapping around the bars used to mount the vanes on the rotor hub as shown in the Martindale patent, with resulting decrease in screening efficiency as well as overloading of the rotor and drive. Similar problems occur with other long fiber suspensions such, for example, as synthetic fibers used in making non-woven fabric, which are frequently of such length as to tend to form strings.